Roll-to-roll solvent annealing of printed P3HT : ICXA devices.

Currently, large-scale roll-to-roll production of printed organic photovoltaics (OPVs) involves high temperature annealing steps that are not compatible with thermally sensitive substrates, such as coated fabrics. In particular, the processing temperatures needed to produce the required crystalline ordering in the printed films are typically above the deformation and melting-points of these substrates. In this paper we investigate the use of local solvent recrystallisation (solvent annealing) on the roll-to-roll scale as a method for avoiding high-temperature thermal annealing. Solvent annealing was performed by slot-die coating a mixture of chloroform and methanol over a previously printed P3HT ICXA active layer film. Peak device performance was found for the 30% chloroform/70% methanol annealing case which increased device performance by a factor of 4 over the not treated devices.

Water-based nanoparticulate solar cells using a diketopyrrolopyrrole donor polymer.

Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active layers have been prepared from a 1 : 2 blend of (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) (PDPP-TNT) and the fullerene acceptor, ([6,6]-phenyl C71-butyric acid methyl ester) (PC70BM). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to investigate the morphology of the active layers of the two approaches. Mild thermal treatment of the NP film is required to promote initial joining of the NPs in order for the devices to function, however the NP structure is retained. Consequently, whereas gross phase segregation of the active layer occurs in the BHJ device spin cast from chloroform, the nanoparticulate approach retains control of the material domain sizes on the length scale of exciton diffusion in the materials. As a result, NP devices are found to generate more than twice the current density of BHJ devices and have a substantially greater overall efficiency. The use of aqueous nanoparticulate dispersions offers a promising approach to control the donor acceptor morphology on the nanoscale with the benefit of environmentally-friendly, solution-based fabrication.

Scanning transmission x-ray microscopy of polymer nanoparticles: probing morphology on sub-10 nm length scales.

Water-processable nanoparticle dispersions of semiconducting polymers offer an attractive approach to the fabrication of organic electronic devices since they offer: (1) control of nanoscale morphology and (2) environmentally friendly fabrication. Although the nature of phase segregation in these polymer nanoparticles is critical to device performance, to date there have been no techniques available to directly determine their intra-particle structure, which consequently has been poorly understood. Here, we present scanning transmission x-ray microscopy (STXM) compositional maps for nanoparticles fabricated from poly(9,9-dioctyl-fluorene-2,7-diyl-co-bis-N, N'-(4-butylphenyl)-bis-N, N'-phenyl-1,4-phenylenedi-amine) (PFB) and poly(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole) (F8BT) 1:1 blend mixtures. The images show distinct phase segregation within the nanoparticles. The compositional data reveals that, within these nanoparticles, PFB and F8BT segregate into a core-shell morphology, with an F8BT-rich core and a PFB-rich shell. Structural modelling demonstrates that the STXM technique is capable of quantifying morphological features on a sub-10 nm length scale; below the spot size of the incident focused x-ray beam. These results have important implications for the development of water-based 'solar paints' fabricated from microemulsions of semiconducting polymers.